1. Field of the Invention
The present invention relates to superconducting magnets provided with an external magnetic-field disturbance compensation coil for compensating external magnetic-field disturbances that cause adverse effects on magnetic resonance imaging for medical use (hereinafter referred to as MRI) employing a magnetic resonance phenomenon occurring in a static magnetic field, and also relates to MRI apparatus using the superconducting magnets.
2. Description of the Prior Art
Several kinds of magnets such as a permanent magnet, a normal conducting magnet, and a superconducting magnet are generally given as a source for a static magnetic field necessary for an MRI apparatus. Among them, a superconducting magnet has been the mainstream of the source due to advantages of major requirements for MRI apparatus such as magnitude and temporal stability of the static magnet field.
However, since a superconducting magnet produces a strong static magnetic field, it is necessary to prevent the magnetic field from leaking out. For the prevention, two methods are mainly employed, by which the magnets, the source of the static magnetic field, are roughly classified into two types.
One of the methods is that a superconducting magnet body is enclosed by a ferrous material that shields the magnet body from the outside thereof (passive shield method). The other is that superconducting main coils of polarities opposite to each other are arranged in pairs, so as to cancel the influence of the produced magnetic field, on its outside (active shield method). Out of the two, a magnet employing the active shield method has been the mainstream due to its lightweight and compactness.
In the meantime, an MRI apparatus is installed at various palaces and in various environments; it is installed next to a road in some cases or close to power cables for power transmission or electric trains in other cases. Under these circumstances, due to approaching of a large ferrous mass or an influence of an alternating magnetic field, magnetic-field fluctuation of non-negligible strength (hereinafter referred to as external magnetic-field disturbance) penetrates from the outside into the imaging space in the static magnetic field while images are being taken. In a case of a superconducting magnet employing the passive shield method, external magnetic-field disturbances often raise no significant problem due to the self-shield effect of the ferrous material. In the case of a superconducting magnet employing the active shield method, however, if the magnet remains intact without taking any measures, most of the external magnetic-field disturbances penetrate into the imaging space, which raises a possibility to exert a considerably adverse effect on the imaging.
Hence, in order to suppress influences of external magnetic-field disturbances, there has been a technique in which a superconducting coil, which is referred to as an external magnetic-field disturbance compensation coil, is arranged independently of the main coils, for exclusively compensating the external magnetic-field disturbances. This is due to the fact that a current is induced in the external magnetic-field disturbance compensation coil to generate a compensating magnetic field when the external magnetic-field disturbances penetrate thereinto. By the compensation with the external magnetic-field disturbance compensation coil and by compensation with the main coil, whose effect is relatively small, magnetic-field fluctuation in the imaging space is suppressed less than a few percent of the amount of the penetrated external magnetic-field disturbances.
Meanwhile, the main coil is a superconducting coil through which a large current passes in a persistent current mode under normal conditions. When a breakage phenomenon of the superconducting state, which is referred to as a quench, occurs for some reason, a large amount of energy is released at a burst. Although most of the energy is released as heat, if the main coil and the external magnetic-field disturbance compensation coil are coupled magnetically with each other, some of the energy is transferred to the external magnetic-field disturbance compensation coil by electromagnetic induction.
At this moment, a current is induced in the external magnetic-field disturbance compensation coil in proportion to the degree of the magnetic coupling. Since the magnetic field generated by the main coil, however, may sometimes not sufficiently decay yet by that moment, the external magnetic-field disturbance compensation coil is subject to extremely large electromagnetic force in many cases. The reason why is that since the external magnetic-field disturbance compensation coil usually has a smaller number of turns than that of the main coil due to such constraints as costs and its installation space, a current larger than that in the main coil (400 to 700 A, for example) might be induced in the external magnetic-field disturbance compensation coil as the case may be.
Moreover, since the overall volume of an external magnetic-field disturbance compensation coil, which is cylindrically wound up, is small due to its small number of turns, a large internal stress is generated in the coil by electromagnetic force. Therefore, it has been difficult to provide sufficient strength for the coil to withstand the electromagnetic force.
For this reason, the external magnetic-field disturbance coil needs to be devised to induce as a low current as possible. For example, a countermeasure is needed in which a current flow would be limited under several dozen amperes. However, since the external magnetic-field disturbance coil is usually made of a superconducting wire, even though using a superconducting wire of low performance, a current of several dozen amperes is likely to pass through the coil. In addition, the reason why the external magnetic-field disturbance coil is made of a superconducting wire is that external magnetic-field disturbances could occur in a situation where a ferrous mass approaches the coil and stays intact there. In such situation, if an external magnetic-field disturbance coil is made of a normal conducting wire such as a copper wire, an induced current immediately decays, which may sometimes not be able to compensate the disturbances for a long time. Therefore, the external magnetic-field disturbance coil should be designed taking such a decay time constant into account.
There has been a configuration that copes with such problems as described above. On detecting a coil quench based upon abnormal voltage on an abnormal voltage detection tap provided on a superconducting coil, a persistent current switch connected in parallel to the superconducting coil is opened to detect an excess voltage via feed lines of the superconducting coil, connected to an excitation power supply. If the voltage exceeds a predetermined value, a circuit breaker works to cut off the current fed from the power supply to the superconducting coil (refer to, for example, Japanese Patent Laid-Open No. 09-260130, paragraphs 0015 through 0019 and FIG. 1 (Patent Document 1)).
There has been another configuration that provides in an external magnetic-field disturbance coil a portion in which a quench occurs at lower current. Even if a large current is induced in the external magnetic-field disturbance coil by a quench in the main coil, the induced current cannot flow any longer due to the quench at the portion (refer to, for example, Japanese Patent Laid-Open No. 04-287903, paragraphs 0049 through 0050 and FIG. 3 (Patent Document 2)).
In order to cut off a power supply current by detecting a quench with a configuration as disclosed in Patent Document 1, it has needed to provide circuits for detecting the quench and shutting off the power, which involves a requirement of a complicated control configuration. Moreover, since an induced current passing through the external magnetic-field disturbance coil is not directly cut off, there has been a problem with response as well.
On the other hand, in a configuration as disclosed in Patent Document 2, there has also been a problem with response, because after a current has been induced in the external magnetic-field disturbance coil by a quench in the main coil, the induced current is not cut off immediately by detecting the quench in the main coil itself, but cut off by quenching the portion of the external magnetic-field disturbance coil by the induced current. In order to provide the portion that is quenched at low current, in part of the external magnetic-field disturbance coil, there is a method of providing a portion that is composed of, for example, one or a few fine (tens μm to hundreds μm) superconducting filaments. Since such filaments, however, have a small heat capacity, malfunctions caused by melting of the filaments are likely to occur due to Joule heat generated after quenching in the filament portion. Moreover, since the filaments are fine, there has been an instability in which the filaments easily break off due to factors other than heat.